Syria's government faced fresh western censure on Monday in the wake of opposition claims of hundreds of people being killed in a new massacre during recent fighting in the Damascus countryside.

William Hague, foreign secretary, called reports of the killings a reminder of the "callous brutality" of the Assad regime and "the terrible climate of impunity" in Syria. He said there was an urgent need to end the 25-month conflict, which has claimed more than 70,000 lives, displaced 2 million Syrians and destabilised the wider region.

The Local Co-ordination Committees, a network of anti-regime activists, said government forces had killed 350 "martyrs" in Artouz, south-west of the capital, on Sunday. Three days earlier 100 people had been killed. The figures could not be independently verified as the area is inaccessible to the handful of foreign journalists and NGOs in Damascus.

The UK-based Syrian Observatory for Human Rights said some victims were summarily killed. Videos posted online showed rows of corpses wrapped in bloody blankets. SOHR said there were reports of as many as 250 dead.

The LCC said there had been clashes between the Free Syrian Army, the main armed opposition group, and the Syrian army's 4th Division, Republican Guard, Lebanese Hezbollah fighters and shabiha militias. Many of the casualties were women, children and elderly people. Corpses were set on fire.

Leaving aside the conflicting casualty figures, the account is consistent with ongoing attempts by government forces to regain control of areas around Damascus as well as two suburbs of the capital city. Syria's official news agency, Sana, reported unspecified heavy losses inflicted on terrorists in Jdaidet al-Fadl, an adjacent area, where rebel sources said 85 people had been killed.

Syrian state TV showed footage of the city with soldiers patrolling the streets and corpses of rebels on the ground. The Revolution Leadership Council of Damascus, an opposition group , said seven people had been killed on Monday by random shelling of a civilian area in Barzeh, north of the city.

But a Syrian military source told the Guardian that troops had moved into Jdaidet al-Fadl to protect civilians and blamed the alleged massacre on rebels.

President Bashar al-Assad, meanwhile, received strong messages of support from his two closest allies.

Alaeddin Boroujerdi, chairman of the foreign policy and national security committee of the Iranian parliament, said on a visit to Damascus that Tehran stood by Syria in its struggle against "the attack targeting it". Assad should remain in power until elections next year, he said.

Sergei Lavrov, Russia's foreign minister, warned the EU not to lift its arms embargo on Syria despite British and French lobbying. His UK counterpart Hague met EU colleagues in Brussels, where they agreed to ease sanctions to allow for purchases of crude oil from the opposition.

If the arms embargo was removed, Lavrov said, "the international obligations of the EU countries, which prohibit supplies of arms and ammunition to non-government actors, are not going anywhere".

Assad's mood is upbeat, according to an account of a meeting on Sunday with Lebanese allies, published in the Beirut daily as-Safir. "Our strategy is to keep Damascus and the other cities under army control," the president said. "As concerns the countryside, we deliberately choose to clear some areas for tactical reasons at times. Better we sap [their strength] than having them sap ours.

"The so-called Free [Syrian] Army is effectively undone. We're now fighting al-Qaida. Some 23 foreign nationalities are currently fighting on Syrian soil. Today, however, we are winning [the hearts and minds] of some of our adversaries. For example, we responded wisely when armed rebels broke into the Yarmouk camp [for Palestinian refugees]. We were urged to force them out. Instead, we reinforced our positions around the camp and fenced in the armed insurgents. In no time, we heard residents clamouring for the terrorists to be evicted. The battle will be very long, but our sole choice is to win it."

Russia has repeatedly used its UN security council veto to protect Assad from western efforts to remove him or pressure him to end violence. Each Russian veto has seen the regime escalate military action, opposition activists say.

Lavrov said in a telephone conversation with Kerry on Saturday: "I sensed confirmation of the intention … to seek as swift as possible a political solution".

Officials of Syria's national opposition coalition, supported by western and Arab countries, say the US is not giving it enough backing and wants to push it into talks with Assad – a position consistent with the outcome of a UN-sponsored conference in Geneva last summer. But few inside Syria appear to believe in the possibility of a negotiated solution to the conflict – unless it is to agree the terms of Assad's departure. The Syrian president has shown no sign he is prepared to step down and action on the ground is outstripping slow-moving diplomacy.